Ethernet network devices, such as network switches and routers, can be classified as data-only devices or data+Power over Ethernet (PoE) devices. A data-only Ethernet network device transfers only data over its Ethernet ports. On the other hand, a data+PoE Ethernet network device (referred to herein as a PoE network device) can transfer both data and power over its Ethernet ports to endpoint devices. In the scenario where a PoE network device acts as a source of power for a given endpoint device, the PoE network device is said to be a PSE (power supplying equipment) and the endpoint device is said to be a PD (powered device). By providing power directly to PDs over Ethernet, PoE network devices can advantageously eliminate the power cabling requirements for such PDs and allow the PDs to be installed at locations that do not have a nearby electrical outlet.
When a PoE network device acts as a PSE for a PD, it takes power from a source (e.g., a power supply connected to the electrical grid or a battery/generator/uninterruptible power supply (UPS)) and supplies the power to the PD over the Ethernet cable to which the PD is attached. In doing this, the PoE network device does not consume more power for its own operation; instead, it transfers the power to the PD. However, this transfer of power is not perfect and is subject to energy losses (mostly within the PoE network device). Typically, 10-25% of the energy drawn by the PoE network device for transfer to PDs is lost during the transfer process. This lost energy is dissipated as heat at the PoE network device. As a result, PoE network devices need to be engineered to dissipate significantly more heat than data-only network devices.
For example, consider an Ethernet switch that comprises 48 ports. Assume that a data-only version of this switch consumes a maximum of 250 watts. In this case, the switch's mechanical and thermal design will be crafted to ensure that the switch can dissipate 250 watts under normal operating conditions.
Now consider a PoE version of the 48-port switch that is configured to deliver 30 watts of power on each switch port. In this case, the switch needs to be able to dissipate the 250 watts consumed for its own operation, as well as the energy losses arising from acting as a PSE for the 48 ports at 30 watts each. These energy losses amount to approximately 48 ports×(30 watts×25%)=360 watts. Thus, for the same 48-port switch, the PoE version needs to be able to dissipate an additional 360 watts over the data-only version.
There are two ways in which a network device can dissipate heat and thereby cool itself: passive cooling and active cooling. With passive cooling, there are no active subsystems (e.g., fans, air conditioners, water pumps, etc.) for dissipating heat from the device; instead, the device is cooled purely by conduction and natural convection. This cooling approach is more reliable than active cooling since there are no moving parts that can fail or break. However, passive cooling generally requires a large device chassis (to promote passive air circulation) and expensive, large-surface area heatsinks, which can significantly increase the size and cost of the device.
In an actively cooled network device, there are typically one or more fans that push/pull air from outside the device to cool the device interior and/or one or more fans that expel hot air from inside the device to the exterior. This allows for higher power density than passive cooling, since the fans provide for significantly more air flow/circulation than possible via natural convection. Thus, for the same number of ports, an actively cooled network device will generally be much smaller (and usually less costly) than a passively cooled network device. However, active cooling also has a number of disadvantages. For example, the fans used to cool the network device must be powered, which increases the power consumption of the device. Further, fans are often very noisy, which can be problematic or unacceptable if the device is deployed in or near an environment where “quiet” is important (e.g., a hotel room, conference room, class room, etc.). Yet further, since fans are mechanical components, they are guaranteed to fail after some period of time. When such active cooling equipment fails, this usually leads to a catastrophic device failure. Thus, the service life and mean time between failures (MTBF) of an actively cooled device are less than that of a passively cooled device. Finally, fans often pull contaminants, such as dust, into the device interior. This can reduce the efficiency of the cooling subsystem over time and, in some cases, can accelerate failure of the device.